Reactive Power Control Of HVDC Systems

With the development of HVDC playing an increasingly important role in modern power system, study of HVDC system operation and control technology has become more and more important. Reactive power control, a significant control part of HVDC system, has relation to the stable operation of HVDC system and close correlation with voltage stability of AC system. Make full use of reactive power regulation capability of HVDC system in favor of meeting different operational needs for AC and DC system.This paper analyzes the mutual cooperation relations between operation law and control function of HVDC, summarizes the basic principles and basic control function of HVDC, then discusses the reactive power control function with pros and cons analysis and application to each sub-function, summarizes the flaws and inadequacies of HVDC reactive power control, and carries out the following studies.Firstly, by analyzing the converter reactive power characteristics and convertor station reactive power control range, it clarifies when the HVDC system is in low power level, convertor station excess reactive power compensation, may increase inverter reactive power consumption by decreasing the transformer tap position and reducing the DC voltage, which can reduce the reactive power exchange between AC and DC system, and in a wide range of reactive power regulation. Similarly, when the HVDC system is in high power level, convertor station lack of reactive power compensation, may reduce the inverter reactive power consumption by increasing the transformer tap position, but reactive power regulation is in a small range.A closed-loop reactive power optimization method is proposed that DC voltage as a control variable and reactive power exchange between AC and DC system as feedback, verified by simulation in CIGRE model with PSCAD/EMTDC. This method overcomes the failure to precise control of reactive power under LLRPO open-loop control, and HVDC instability with the direct control rectifier firing angle or inverter extinction angle under QPC control.Then, applied it to real HVDC project, meanwhile, considered the impact of the converter transformer tap position, with the use of RTDS model, it improves that decrease converter transformer tap position could relieve the overabundance of reactive power in convertor station when HVDC system is in low power level, and increase converter transformer tap position could relieve the shortage of reactive power in convertor station when HVDC system is in high power level.Finally, HVDC system reactive power control is summarized and expected.